Law enforcement officials and traffic safety experts fear that marijuana's rising popularity will further spike the traffic hazard of driving while high.

While research remains incomplete on how much toking is too much, "smoking marijuana has a very negative effect on your ability to operate a motor vehicle,'' said Gil Kerlikowske, director of the White House Office of National Drug Control Policy - the Drug Czar. "It's quite dangerous to you, your passengers and others on the road.''

Marijuana advocates acknowledge that driving under the influence of cannabis is ill-advised. But they argue that law enforcement's concern is overblown and point to a study last year that concluded the auto accident risk posed by marijuana is on par with antihistamines and penicillin.

Jefferson County Sheriff's Office spokesman Rod Carroll said under Texas law, it doesn't really matter what substance a driver is on where law enforcement is concerned. The state's statutes are intentionally vague to serve as a catch-all for anyone intoxicated behind the wheel, regardless of the intoxicant.

The debate over marijuana and highway safety is set against the backdrop of voter-approved marijuana legalization - for personal use - in Washington and Colorado, as well as medical marijuana laws on the books in California and 19 other states.

"I'm not sure the public really understands the danger of it,'' said Chuck Hayes, a retired Oregon State Police captain who works with the International Association of Chiefs of Police training uniformed officers to be drug recognition experts. "A lot of education needs to be done in this area.''

Texas has embraced neither legalization nor medical marijuana. Yet law enforcement officials express concern that marijuana and other drugs present more and more of a hazard on Texas highways.

"Driving is the most dangerous thing we do on a routine basis,'' said Texas Department of Public Safety Director Steven McCraw in an interview. "Anything you do that changes your perception, reaction times, reasoning (and) alertness is a threat to public safety, no question.''

McCraw said he's particularly worried by the appeal of marijuana, "ecstasy'' and other drugs to teens - "the most at-risk driving population.''

Traffic fatalities have been trending upward over the past three years, with 2,423 killed in auto accidents so far this year.

Texas does not separate drugged driving from drunk driving, so it is difficult to measure the impact of drugs on traffic safety.

But of 62,962 police requests to DPS in the last three years for blood-alcohol concentration levels - the standard for measuring drunk driving - almost a third of them (19,366) also required toxicology exams for drugs.

Such exams currently measure for cocaine, methamphetamine and other drugs but not marijuana, according to DPS communications director Katherine Cesinger. But marijuana screening will be available "later this year,'' she said.

Jefferson County's Carroll said it's difficult to tell if there's been a recent fluctuation locally in what drugs are coursing through the veins of Southeast Texas' intoxicated drivers, though there is one thing that's clear.

"We've always had it," he said of driving under the influence. "It's always been here."

In the 1990s, the widespread availability of prescription pills became evident to patrol and highway officers, who ran into drivers sporting what he called "blown pupils."

Expanded pupils, which obscure the natural color of someone's eyes, are generally considered a telltale sign that someone is intoxicated and something officers notice quickly when talking with a stopped driver.

Each of the 2,600 troopers who patrol Texas highways has training on how to recognize whether a driver is under the influence of drugs. In addition, 95 of the troopers plus 346 other police officers in Texas have completed the rigorous course to become certified as "drug recognition experts.''

Numerous academic studies have concluded there is clear evidence of a link between marijuana consumption and traffic accidents.

A study conducted last year at Dalhousie University Medical School in Canada found those who consume cannabis within three hours of driving are almost twice as likely to cause an accident as those who are drug-or alcohol-free.

Since marijuana's active ingredient, THC, lingers in blood and urine for days after consumptions, users who are not high might end up under arrest for relatively minor traffic violations, such as failure to use a turn signal or not wearing a seat belt.

"Marijuana has been in use for decades without significant risk on the roads,'' said Kris Hermes, spokesman for Americans United for Safe Access, a medical marijuana advocacy group. "We don't need to suddenly protect the public from a problem that doesn't exist.''

Kerlikowske, a former Seattle police chief who is President Obama's nominee to head U.S. Customs and Border Protection, called claims about detrimental impact on marijuana users "a bit of a red herring.''